There has long been a need for rapidly and accurately sensing the angular orientation of lines or slits on manufactured articles, such as microcircuits, optical masks, semiconductors, grills, gratings, meshes, patterns, and the like, without actual physical contact, for such purposes as quality control, control of the manufacturing process, and assembly line orientation of the article. It will be understood that the terms "line" and "slit," as used in this specification and claims, include both continuous and discontinuous straight lines or slits. In the latter case, it is essential only that the centers of discontinuous components making up the "line" or "slit" be arranged along the same longitudinal axis and the fringe period be substantially wider than the widest component, or the positive difference between the width of the component and a whole integer multiple of the fring period. Within these limitations, the components can be of any desired shape and size, e.g., rectangular, triangular, trapezoidal, round, oval, and the like. The line can also be a line defining an edge of the article.
Monitoring of angular orientation on manufactured articles is generally accomplished by such manual and visual means as superimposing an image on a standard or master. Such methods are laborious, tiring, and are subject to human error. Another method, which is more accurate but is very expensive and complex, involves the use of an image which is converted to electronic signals and compared by a computer with a master image. The rotation of the monitored image must be accomplished in very small increments, each of which must be compared by the computer. In some cases, no reliable or feasible automatic method has hitherto been available.
Laser Doppler Velocimeters (LDV) have recently been developed for determining the rate of fluid flow in wind and water tunnels by suspending small particles in the fluid and determining their velocity and size by means of the velocimeter. Such velocimeters generally comprise convergent laser beams of equal size, intensity, and frequency which produce a stationary interference fringe pattern within the zone of convergence, sometimes called the probe volume. The interference fringes are planes which are normal to the plane defined by the center lines of the two converging laser beams and parallel to the bisector of the converging beams. In operation, the apparatus is set up so that the fluid-borne particles move across the fringes in a plane normal to the fringe planes, the radiation scattered by the moving particles is optically collected, separated electronically into AC and DC signal components, and the AC/DC ratio is used as a means of determining the size of the particles. Such Laser Doppler Velocimeters are described in detail in the article by W. M. Farmer, "Measurement of Particle Size, Number Density, and Velocity Using a Laser Interferometer," Applied Optics, Vol. 11, No. 11, Nov. 1972, pp. 2603-2612, and G. J. Rudd, U.S. Pat. No. 3,680,961.
In more recent development of the Laser Doppler Velocimeter, the art discloses the use of probe volumes in which the fringes are caused to move continuously in a direction normal to the fringe planes by employing converging laser beams of the same intensity but slightly different frequency, the frequency difference .DELTA.f being within the radio frequency band. Such shifting of the frequency of one of the beams can, for example, be produced by diffraction of an incident laser beam by means of an ultrasonic Bragg cell, which can be made to divide the incident beam into two diverging beam components of the same intensity, one nondiffracted component having the incident beam frequency and the other diffracted component with its frequency shifted by an amount equal to the Bragg cell frequency. Since the two coherent light beams which leave the Bragg cell are diverging, it is required that the beams be converged by an appropriate optical system to form the desired interference fringe pattern. The moving fringe pattern moves at a rate equal to .DELTA.f which in turn is equal to the Bragg cell frequency.
The moving fringe technique has been applied to the LDV primarily to provide a means for determining the direction of movement of the particles moving across the fringe planes. It provides no improvement in determination of particle size. The application of single and two-dimensional Bragg cell systems to the LDV is disclosed in Chu et al, "Bragg Diffraction of Light by Two Orthogonal Ultrasonic Waves in Water," Appl. Phys. Lett., Vol. 22, No. 11, 1 June 1973, pp. 557-59; and W. M. Farmer et al, "Two-Component, Self-Aligning Laser Vector Velocimeter," Applied Optics, Vol. 12, No. 11, Nov. 1973, pp. 2636-2640.
None of the available art recognizes or discloses the present invention, its principle of operation, or its use for sensing the angular orientation of line or slit elements of an article. The present invention utilizes known fringe spacings (which can be calculated or otherwise determined by conventional art techniques) and rotation of the fringe zone and article relative to each other to determine the angle of orientation of line or slit elements on the article. Any observed deviation of angular orientation can, by means of appropriate conventional electronics, be employed, either in a simple display showing the deviation or as a feedback means for regulating the article production process.
Because the measurement does not require absolute measurement of laser light intensity, but only the detection of angular position(s) where maximum occurs, the process and apparatus of the invention have additional advantages including but not limited to the following. Accuracy of measurement is largely independent of intensity fluctuations of the laser source. Accuracy is not affected or compromised by the reflectivity or refractivity of the line elements. Accuracy does not depend on the calibration accuracy of the signal detector devices or the distortions or nonlinearities of components of the optical system, either per se or in terms of sensitivity to changing environmental conditions. Thus, the system and components can be relatively low-cost and can be used in uncontrolled environments, such as manufacturing facilities.